Many modern electronic devices operate at clock frequencies high enough to generate substantial electromagnetic interference. Moreover, the electromagnetic interference generated by such an electronic device may adversely affect operation of another electronic device in the same vicinity. In particular, the electromagnetic interference generated by an electronic device may significantly degrade performance of an electrical circuit and adversely impact operation of electronics equipment in the vicinity. Thus, reduction of electromagnetic interference is often a criterion for designing an electronic device. Moreover, an electronic device is typically designed so that electromagnetic interference emitted from the electronic device does not exceed limits regulated by the Federal Communications Commission (FCC).
One technique for reducing electromagnetic interference emitted from electronic devices involves reducing the peak radiated energy of a clock signal by spreading the frequency of the clock signal over a frequency band. As a result, the energy of the clock signal is spread over the frequency band. Spreading the frequency of the clock signal prevents the energy of the clock signal from concentrating at a single frequency and its harmonics which could otherwise cause electromagnetic interference in the clock signal to exceed regulatory limits. Although this technique has been successfully employed to reduce electromagnetic interference in some types of electronic devices, this technique may cause artifacts to occur in a video image displayed by using a video clock signal of the electronic device. In particular, spreading the frequency of the video clock signal may cause unwanted row noise (e.g., extraneous horizontal lines) to occur in the video image.
One spread spectrum technique for reducing electromagnetic interference in a video clock signal of an electronic device involves modulating the video clock signal with a horizontal sync signal of a video display in the electronic device. Modulating the video clock signal with the horizontal sync signal spreads the frequency of the video clock signal and reduces the occurrence of artifacts in a video image displayed by the electronic device. Although this technique has been successfully employed to reduce electromagnetic interference in video clock signals of some types of electronic devices, this technique is sensitive to voltage and temperature variations across an electronic device which reduces the effectiveness of this technique.
In light of the above, a need exists for an improved system and method of reducing electromagnetic interference in a video clock signal of an electronic device. A further need exists for effectively reducing electromagnetic interference in a video clock signal of an electronic device across temperature and voltage variations in the electronic device.